Sorry, just to clarify and not mislead. The hard rubber ring is embedded and visible on the outside of the pulley facing toward the car. When I said insert, I meant the hole that the bolt goes thru...nothing there.
BTW--the ring itelf on my pulley, was all brittle and chunks of it were missing, when I installed an AP pulley. Anyway, not sure how helpful it could be at that point or balanced for that matter.